1. Field of the Invention
The present invention relates to a solid polyelectrolyte-type fuel cell.
2. Discussion of the Background
A solid polyelectrolyte-type fuel cell is considered to be a hopeful, small-sized lightweight power source for vehicles and other devices, in which hydrogen and oxygen are used as the fuel. The cell comprises an ion-exchangeable, solid polyelectrolyte membrane, and positive and negative electrodes disposed to be in contact with both sides of the membrane. The hydrogen fuel is electrochemically oxidized at the negative electrode to give protons and electrons. The protons pass through the polyelectrolyte membrane toward the positive electrode to which oxygen is fed. Electrons having been formed at the negative electrode, travel to the positive electrode, where the protons and the electrons react with oxygen to form water.
The solid electrolyte-type fuel cell can operate at low temperatures and is small-sized, while producing a high output density. Therefore, many studies have been made on these types of cells for use as the power source for vehicles. In the cell, sulfonic acid group-containing perfluorocarbon polymer membranes (e.g., NAFION, trade name of DUPONT Co.; ACIPLEX, trade name of ASAHI CHEMICAL Co.) or the like have been generally used as the polyelectrolyte membrane. However, the conventional fuel cell is not still satisfactory as its output is too low.
In order to increase the output of the cell, the hydrogen ion conductivity of the solid polyelectrolyte membrane therein must be increased to lower the internal resistance of the cell. For this, the concentration of the ion-exchanging groups (for example, sulfonic acid group) in the solid polyelectrolyte membrane may be increased and the thickness of the membrane may be reduced. However, too great an increase in the ion-exchanging group concentration in the membrane results in an increase in the water content of the membrane to an undesirable degree, and is therefore problematic in that the positive electrode at which water is formed through the cell reaction becomes too wet, lowering the cell output.
On the other hand, a reduction in the thickness of the membrane is also problematic in that the mechanical strength of the membrane is reduced and the amount of the fuel (hydrogen gas and oxygen gas) passing through the membrane is increased, lowering the cell-out efficiency.
In order to solve these problems, Japanese Patent Application Laid-Open (JP-A) Hei-6-231780 proposed a casting method comprising infiltrating a sulfone-type perfluorocarbon polymer into woven fabric of polytetrafluoroethylene followed by drying and filming it around the fabric; and a method comprising hot-melting a sulfone-type perfluorocarbon polymer on woven fabric of polytetrafluoroethylene under pressure. In these methods, the object was to reinforce the polymer film.
However, in the casting method, adhesion between the woven fabric and the sulfone-type perfluorocarbon polymer is weak; and in the pressure hot-melting method, the fabric and the sulfone-type perfluorocarbon polymer are only partially fused and adhered together, but are not completely integrated. In JP-A Hei-6-23 1780, fibrils of polytetrafluoroethylene are mixed with a sulfone-type perfluorocarbon polymer and extruded into sheets, in place of using woven fabric of polytetrafluoroethylene. In this method, however, the melting point of polytetrafluoroethylene is so high that the sulfone-type perfluorocarbon polymer mixed with its fibrils could not be completely fused and integrated.
When the membranes as produced according to the proposed methods are used in fuel cells, the sulfone-type perfluorocarbon polymer is separated from the fibrils or woven fabric of polytetrafluoroethylene while the cells are driven, resulting in the membranes not maintaining their initial mechanical strength. In addition, where the woven fabric is used as the reinforcing material, its thickness is often uneven, and the reduction in its thickness is limited.